1. Field of the Invention
The present invention generally relates to a structural member having a pipe shape and more particularly, to a pipe-shaped structural member adapted to behave in such a peculiar style of movement as to twist when it is bent, while bending when it is twisted, while utilizing mechanical anisotropy of FRP (fiber reinforced plastics) and/or fiber reinforced rubber. The structural member of the present invention is applicable to a golf club shaft, a robot arm for mechanical work, a pipe structure used in the space industrial field or used in toys and the like.
2. Description of the Prior Art
A conventional structural member having a pipe shape and made of an isotropic material such as iron, aluminum, etc. deflects, but does not twist when it is bent by a load applied to a point on the geometric principal axis. On the other hand, it not only deflects but also twists when it is bent and twisted by a load applied to a point not on the geometric principal axis.
More specifically, as shown in FIGS. 61 and 62, if a pipe-shaped structural member 1 made of an isotropic material is applied with a load to the free end 1b thereof, 1a being the fixed end, in a direction shown by an arrow A so that a line of action intersects the geometric principal axis G of the pipe-shaped member, the member 1 deflects as indicated by a chain line in the drawings, without twisting.
In contrast, with reference to FIGS. 63 and 64, if the pipe-shaped structural member 1 is applied with any load to one point at the free end 1b in a direction shown by an arrow B so that the line of action does not cross the geometric principal axis G, the pipe-shaped structural member 1 deflects and twists as indicated by a chain line in the drawings.
Although the pipe-shaped structural member made of isotropic material is conventionally designed to act in the aforementioned manner, the movement thereof is restricted and no such peculiar action is realized whereby it twists when it is bent and it bends when it is twisted, and moreover it deflects without twisting when it is bent and twisted.
In the meantime, anisotropic material, for example, FRP (fiber reinforced plastics) can achieve the mechanical characteristics not realized by the isotropic material if the direction of fibers therein is controlled.
The FRP has been used hithertofore through combination of the other properties thereof other than the mechanical property, e.g., high rigidity and high modulus of elasticity with the thermodynamic, electric or chemical properties of each component. Moreover, although the FRP has been used with an aim to achieve a product of light weight, the mechanical property of the FRP as an anisotropic material has hardly been applied to a positive use.
On the other hand, a fiber reinforced rubber has been proposed as a material with mechanical anisotropy. In comparison with the FRP, the fiber reinforced rubber is low in rigidity and elasticity, so that it is easily deformed and greatly expanded, even by a small force until it ruptures. An orientated rubber shows the same mechanical anisotropy as the fiber reinforced rubber. However, the mechanical property of the orientated rubber and the fiber reinforced rubber has nor been positively utilized.
As described hereinabove, the geometric principal axis and elastic principal axis of the conventional pipe-shaped structural member agree with each other. Therefore, in the event such a pipe-shaped structural member is subjected to a bending and twisting load onto the elastic principal axis not coincident with the geometric principal axis, it undesirably causes the member to twist and deflect.
Now, taking a conventional golf club shaft by way of example, as indicated in FIG. 65, the part of a golf club shaft 101 where a grip 102 is attached becomes a fixed end when the player holds the grip 102, and the end of the shaft 101 where a club head 103 is mounted becomes the free end. The elastic principal axis E1 at the free end of the club shaft 101 is coincident with the geometric principal axis G.
Referring further to FIG. 66, in general, the center of gravity A of the weight (hereinafter referred to simply as the center of gravity) of the club head 103 and the geometric center of gravity B of a scoring area 104 indicated by oblique lines on a club face 103a are separated about 20-50 mm away from the geometric principal axis G of the club shaft 101. In other words, the position of the elastic principal axis E1 at the front end of the club shaft 101 is neither coincident with the center of gravity A of the club head, nor with the geometric center of gravity B of the scoring area 104 in the conventional golf club shaft as the player holds the grip 102.
Since the center of gravity of the club head is conventionally not found on the geometric principal axis as mentioned above, the shaft 101 is applied with a bending and twisting moment resulting from the inertia force, on condition that the shaft is added with the eccentric load when the player swings the shaft. As a result, the golf club shaft 101 is not only bent but twisted. Similarly, when the player hits the ball, the repulsive force from the golf ball to the scoring area 104 of the club head 103 gives such moment to the shaft 101 that leads to bending and twisting of the shaft.
Consequent to the bending and twisting of the shaft when the player swings or makes a strike, the face 103a of the club head 103 is rotated to the geometric principal axis G of the shaft 101, and the golf ball is shifted from the intended direction even when the face 103a is turned correctly to the golf ball. Therefore, the player hardly exerts a correct command of direction of the golf ball.
In the meantime, a conventional robot arm 201, for example as shown in FIG. 67, has its fixed end 201a secured to a working body 203 of a robot main body 202. The working body 203 performs a rotating movement (indicated by an arrow C) and a parallel movement (indicated by an arrow D). At the same time, a free end 201b of the robot arm 201 is equipped with a mounting member 205 which is a robot hand for holding a to-be-transferred object 204. The elastic principal axis E2 at the free end 201b agrees with the geometric principal axis G.
However, the center of gravity F of the mounting member 205 itself or the center of gravity J when the mounting member 205 holds the object 204 is not present on the geometric principal axis G. As such, since the mounting member 205 of the conventional robot arm 201 has the center of gravity F not on the geometric principal axis, when the robot arm 201 does not hold the object 204, i.e., when the robot arm 201 starts rotating movement or parallel movement to hold the object, the inertia force of the mounting member 205 works as an eccentric load acting on the center of gravity F, thereby adding a bending and twisting moment to the robot arm 201. As a result, the robot arm 201 is deflected and twisted while generating vibrations when it starts or stops the movement. Moreover, also when the mounting member 205 moves while holding the object 204, similar to the above case, since the center of gravity J of the member 205 and object 204 is not on the geometric principal axis G, vibrations due to the deflection and twist of the member 205 are brought about when the rotating movement or parallel movement is started or stopped.
These vibrations resulting from a deflection and distortion of the robot arm 201 adversely affects the accuracy of the stopping position of the arm, making it impossible to hold or position the object 204 correctly at a predetermined position. Particularly, the above-mentioned vibrations give adverse effects to the robot arm with low rigidity or in the case where high accuracy or high speed is required.